The actual paper presents the findings differently. To quote:

Our results clearly indicate that the resolution limit of the eye is higher than broadly assumed in the industry

They go on to use the iPhone 15 (461ppi) as an example, saying that at 35cm (1.15 feet) it has an effective “pixels per degree” of 65, compared to “individual values as high as 120 ppd” in their human perception measurements. You’d need the equivalent of an iPhone 15 at 850ppi to hit that, which would be a tiny bit over 2160p/UHD.

Honestly, that seems reasonable to me. It matches my intuition and experience that for smartphones, 8K would be overkill, and 4K is a marginal but noticeable upgrade from 1440p.

If you’re sitting the average 2.5 meters away from a 44-inch set, a simple Quad HD (QHD) display already packs more detail than your eye can possibly distinguish

Three paragraphs in and they’ve moved the goalposts from HD (1080p) to 1440p. :/ Anyway, I agree that 2.5 meters is generally too far from a 44" 4K TV. At that distance you should think about stepping up a size or two. Especially if you’re a gamer. You don’t want to deal with tiny UI text.

It’s also worth noting that for film, contrast is typically not that high, so the difference between resolutions will be less noticeable — if you are comparing videos with similar bitrates. If we’re talking about Netflix or YouTube or whatever, they compress the hell out of their streams, so you will definitely notice the difference if only by virtue of the different bitrates. You’d be much harder-pressed to spot the difference between a 1080p Bluray and a 4K Bluray, because 1080p Blurays already use a sufficiently high bitrate.

I think people, and this paper, misses a few elements.

4K encoded content often has significantly higher bitrate (well, duh, there’s more content) and often higher than the simple increase in pixel density would suggest. So content with heavy moment (flocks of birds, water, crowds etc) still looks better than 1080p, not because of the increase in pixel density, but because of the decrease of compression artefacts.

Second, high dynamic range yo! On a still picture on my TV it’s hard to see difference between 1080p and 4K but it isn’t hard to see the difference between SDR and HDR.

So I still vastly prefer 4K content, but not because of the resolution.

And I’m still here with a 1080p TV. Because my previous ISP had data caps, I’m not convinced my current ISP won’t move to data caps, and there isn’t a lot of good 1080p content out there besides DVD/BluRay and my Rips, let alone 5k or 8k content.

1080p to 4k was a big improvement in my opinion. I still have a mix of 1080p and 4k equipment. 4k to 8k is real minor to me because 4k TVs are varying degrees of HDR now. Brightness range per zone/pixel and wider color gamut. 8K someday because someday the only TVs you should be buying for the price will be 4k but content picture quality, 4k with quality HDR brightness, contrast, color gamut - minor difference. Just need high quality sources. When I encode something, I use fairly high bitrate AV1. Another 5-7 years and I expect to be encoding everything new at AV2

Depends on the source material, the distance you look and quality of the panel. 4k is just more resolution than HD. This does not really need a study.